1. Field of the Invention
The present invention relates to a bulb for a discharge lamp used as a vehicle lighting device. More particularly, the present invention relates to a bulb for a discharge lamp, with improved durability due to a specific water content or pressure of gas enclosed in a hermetically sealed space between an arc tube including a light emitting portion-of a discharge bulb and a shroud glass tube surrounding the arc tube.
2. Description of the Related Art
Recently, discharge bulbs have been used as vehicle lighting devices such as head lamps for automobiles. Typically, discharge lamps having the following structure have been employed. Light is emitted by a discharge phenomenon between electrodes oppositely arranged in a glass bulb, in which a xenon gas is enclosed.
The structure of the discharge bulb will be schematically described below. First, discharge electrodes made of tungsten are oppositely arranged in a sealed space (sealed chamber) obtained by pinch-sealing a long, thin, glass tube at a predetermined interval, so as to form an arc tube including a spherical light emitting portion provided therein. The light emitting portion is filled with a starting gas (xenon gas), mercury and metal halide (hereinafter, referred to as xe2x80x9clight emitting substancexe2x80x9d).
In order to cut an ultraviolet component-having a wavelength harmful to human eyes-from the light emitted from the light emitting portion, a substantially cylindrical shroud glass tube is provided so as to seal and surround the arc tube. Conventionally, in the discharge bulb, an atmospheric air has been enclosed in a sealed space between the arc tube and the shroud glass tube.
However, the conventional art has the following technical problems.
(1) In the case where an atmospheric air containing much water is enclosed in the sealed space between the arc tube and the shroud glass tube, when the discharge bulb is repeatedly turned on and off, a temperature remarkably varies in the sealed space. For this reason, the sealed space easily becomes dewy. When the sealed space is dewy, a capillary condensation of water content gradually occurs in a narrow gap between the sphere-shaped, bulged, light emitting portion of the arc tube and the shroud glass tube. As a result, disadvantageously, devitrification (whitening) and expansion of the light-emitting portion easily occur. In particular, when glass is formed, a gas containing water content more than the atmospheric air is exhausted from a burner and is enclosed within the sealed glass tube, whereby a critical problem arises.
(2) A xenon gas is usually enclosed in the light emitting portion at about 5 to 10 atmospheres pressure and, thus, an internal pressure of the light emitting portion arrives at several tens of atmospheres pressure when the discharge bulb is turned on. For this reason, when the turn-on time becomes long, the light emitting portion is gradually expanded and, then, closely approaches an inner wall surface of the shroud glass tube. As a result of the above, devitrification is generated. Moreover, the light emitting portion contacts with the inner wall surface of the shroud glass tube, resulting in leakage or breakdown.
The present invention has been made in view of the problems in the prior art. It is, therefore, an object of the present invention to improve durability of discharge bulbs by specifying a predetermined range of water content or pressure for gas enclosed in a sealed space between an arc tube of the discharge bulb and a shroud glass tube surrounding the arc tube.
In order to achieve the above and other objects, the following aspects of the invention have been employed.
More specifically, according to a first aspect, the present invention provides a discharge bulb including: an arc tube having a light emitting portion constructed in a manner that a light emitting substance or the like is enclosed therein by pinch-sealing the arc tube, and discharge electrodes are oppositely arranged therein; and a shroud glass tube hermetically sealing and covering the arc tube so as to form a sealed space between the shroud glass tube and the arc tube, wherein a water content of the gas enclosed in the sealed space is set to less than 130 ppm.
In this aspect, since the water content of air existing in the sealed space is made low, the dew point becomes less than xe2x88x9240xc2x0 C. Therefore, even if the discharge bulb is repeatedly turned on and off, the sealed space does not easily become dewy, and there is no possibility of facilitating devitrification (whitening) or expansion of the light emitting portion by capillary condensation of the water content generated in a narrow space between the light emitting portion of the arc tube and the shroud glass tube.
Further, according to a second aspect, the present invention provides the discharge bulb according to the first aspect, but is further characterized in that the light emitting portion is formed so as to closely approach an inner wall surface of the shroud glass tube, and the sealed space is filled with a gas within a range from a lower atmospheric pressure limit, calculated by 3xe2x88x926 d, to an upper limit of 15 atmospheres pressure, wherein a distance from the inner wall surface of the shroud glass tube to a zenith portion of the light emitting portion is set as reference numeral d (in the unit of mm).
In this aspect, the distance d from the inner wall surface of the shroud glass tube to the zenith portion of the light emitting portion is set to a proper value, and a pressure of the sealed space is specified. By doing so, it is possible to reduce a generation of devitrification, leak, and breakdown, of the light-emitting portion. Moreover, the light emitting portion does not reach an abnormally high temperature by the thermally conductive effect of the gas. Therefore, it is possible to prevent the glass from being softened and expanded.
In this case, the calculating equation xe2x80x9c3xe2x88x926dxe2x80x9dxe2x80x94for determining the lower atmospheric pressure limitxe2x80x94is obtained from an experiment conducted in order to obtain a relation between an expansion length of the light emitting portion and a pressure of the sealed space when turning on the discharge bulb.
Further, according to a third aspect, the present invention provides a manufacturing method of a discharge bulb having (i) an arc tube having a light emitting portion constructed in a manner that a light emitting substance or the like is enclosed therein by pinch-sealing a glass tube, and discharge electrodes are oppositely arranged, and (ii) a shroud glass tube hermetically sealing and covering the arc tube so as to form a space between the shroud glass tube and the arc tube, wherein the method includes:
a gas filing process for filling a gas into the space, wherein the gas filling process includes a gas introducing process for introducing a gas having a specified water content of less than 130 ppm into the space; and
a sealing process for sealing the shroud glass tube so as to seal the space.
In this aspect, by specifying the water content of the sealed space, it is possible to manufacture a discharge bulb that does not easily become dewy, even when the discharge bulb is repeatedly turned on and off.
Further, according to a fourth aspect, the present invention provides the manufacturing method of a discharge bulb according to the third aspect, but further characterized in that the gas introducing process is carried out within a range from 0.3 to 15 atmospheres pressure. By doing so, it is possible to manufacture a discharge bulb that can securely prevent an expansion of the light emitting portion.
Further, according to a fifth aspect, the present invention provides the manufacturing method of a discharge bulb according to either the third or fourth aspects, but further characterized in that the sealing process is carried out so that the shroud glass tube is cooled, whereby the gas is liquefied. By doing so, it is possible to fill the tube with gas having 1 atmosphere pressure or more.
As described above, the present invention improves durability (long life) of a discharge bulb. That is, the present invention contributes to improvement in the quality of discharge bulbs.